Sintered cobalt-neodymium-samarium intermetallic product and permanent magnets produced therefrom

ABSTRACT

Novel sintered cobalt-rare earth intermetallic products are used to form permanent magnets having unique properties. The sintered product is comprised of intermetallic compounds of cobalt and rare earth metals composed of samarium and neodymium. Cobalt is present in an amount of about 62 to 66 percent by weight of the product and the rare earth metals are present in an amount of about 34 to 38 percent by weight of the product with the neodymium component ranging in amount from about 20 to 90 percent by weight of the rare earth content. Permanent magnets are formed from the sintered product in bulk form or in particulate form.

United States Patent [191 Martin 1111 3,821,035 [451 June 28, 1974 1 SINTERED COBALT-NEODYMIUM-SAMARIUM INTERMETALLIC PRODUCT AND PERMANENT MAGNETS PRODUCED THEREFROM [75] Inventor: Donald L. Martin, Elnora, NY. [73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: May 1, 1972 [211 Appl. No.: 249,362

[52] US. Cl. 148/3157, 148/101, 148/103, 1

1 148/105 [51] Int. Cl. H0lf 1/04 [58] Field of Search 148/101, 103, 31.57;

[56] References Cited UNITED STATES PATENTS 3,424,578 l/l969 Strnat et al. 148/101 3,546,030 12/1970 Buschow et 211.. 148/3157 3,655,463 4/1972 Benz 148/103 3,682,714 8/1972 Marti 148/3157 3,682,715 8/1972 Martin 148/3157 3 ,682,716 8/1972 Martin et 148/3157 3,684,591 8/1972 Martin 148/101 3,684,593 8/1972 Benz et 211 148/3157 3,695,945 10/1972 Benz 148/103 OTHER PUBLICATIONS Tsui, J. et al.; Evaluatn. of Utility of (Pr,1 Id)Co Alloy for Perm. Mags.;-in Jou'm. Appl. Phys, 42 (1971) pp. 15391540.

Buschow, K. et al.; Permanent Magnet Materials of Re-Co Compounds; in Zeit. for Aug. Pysik; New York, 1969 pp 157-160.

Velge, W. et a1; Permanent Magnet Properties of Re-Co Compounds; in Proc. I.E.E.E. Conf.; London, 1967-pp 45-50.

Primary Examiner-Wa1ter Satterfield Attorney, Agent, or Firm-Jane M. Binkowski; Joseph T. Cohen; Jerome C. Squillaro [57] 1 ABSTRACT Novel sintered cobalt-rare earth intermetallic products are used to form permanent magnets having unique properties. The sintered product is comprised of intermetallic compounds of cobalt and rare earth metals composed of Samarium and neodymium.- Cobalt is present in an amount of about 62 to 66 percent by weight of the product and the rare earth metals are present in an amount of about 34 to 38 percent by weight of the product with the neodymium component ranging in amount from about 20 to 90 percent by weight of the rare earthcontent. Permanent magnets are formed from the sintered product in bulk form or in particulate form.

1 Claim, 2 Drawing Figures SINTERED COBALT-NEODYMIUM-SAMARIUM .INTERMETALLIC PRODUCT AND PERMANENT MAGNETS PRODUCED THEREFROM The present invention relates generally to the art of permanent magnets and is morefparticularly concerned with novel sintered cobalt-neodymium-samarium intermetallic products having unique characteristics and with permanent magnets formed therefrom.

Permanent magnets, i.e., hard magnetic materials such as the cobalbrare earth intermetallic compounds, are of technological importance because they can maintain a high, constant magnetic flux in the absence of an exciting magnetic field or electrical current to bring about such a field.

Cobalt-rare earth intermetallic compounds exist in a variety of phases. The permanent magnet properties of cobalt-rare earth intermetallic magnetic materials generally can be enhanced by reducing the bulk bodies to powders, but in such finely-divided form these materials are unstable in air and their magnetic properties deteriorate after a short period of time.

One object of the present invention is to provide novel cobaltrare earth intermetallic magnets which utilize the rare earth neodymium. Specifically, Co Nd magnets have a high saturation induction but poor coercive force which makes them generally unsuitable for wide applications. By the present invention, permanent magnets are provided which contain neodymium in a significant amount and which have good permanent magnet properties including a satisfactory coercive force H Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with the figures accompanying and forming a part of the specification, in which:

FIG. 1 is the cobalt-samarium phase diagram. It is assumed herein that each phase diagram at 300C, which is the lowest temperature shown in the figure, is substantially the same at room temperatures.

FIG. 2 is a chart bearing curves which illustrates the magnetic properties of permanent magnets produced in accordance with the present invention. Specifically, it illustrates what positive values of magnetization can be maintained in the presence of the demagnetizing field H.

Briefly stated, the sintered product of the present invention is comprised of interrnetallic compounds of cobalt and rare earth metals composed of samarium and neodymium. Cobalt is present in an amount of about 62 to 66 percent by weight of the product and the rare earth metals are present in an amount of about 34 to 38 percent by weight of the product with the neodymium component ranging in amount from to 90 percent by weight of the rare earth content with the preferred amount being 40 to 60 percent by weight of the rare earth content. Permanent magnets are formed from the sintered product in bulk form or in particulate form.

The sintered product of the present invention may be produced in a variety of different ways but I prefer to use substantially the process disclosed and claimed in US. PatQNo. 3,655,465 filed in the name of Mark G. Benz, and assigned to the assignee hereof, and which by reference is made part of the disclosure of the present application. Briefly stated, the process of US. Pat. No.

3,655,464 comprises the steps of forming a particulate mixture of a base cobalt-rare earth alloy and additive cobalt-rare earth alloy. The base alloy is one which at sintering temperature exists as a solid Co R intermetallic single phase where R is a rare earth metal. The addi- V body which is sintered to the desired density and phase composition. In the present invention, the final sintered product contains a major amount of the Co R intermetallic phase and a minor amount of the Co R phase. Generally, the Co R phase is present in an amount ranging from about 0.1 to 5 percent by weight of the sintered product. p

The sintered product of my invention is also suitably produced by using substantially the process disclosed and claimed in US. Pat. No. 3,655,463 filed in the name of Mark G. Benz and assigned to the assignee hereof, and which by reference is made part of a disclosure of the present application.

The procedure for forming sintered products disclosed in US. Pat. No. 3,655,463 is substantially the same as that disclosed in US. Pat. No. 3,655,464 except that an additive Co-R alloy which is solid at sintering temperature and which is richer in rare earth metal than the base alloy is used.

As applied to the preparation of thenew products of the present invention, the process is carried out with a base alloy which is solid at sintering temperature and which at sintering temperature is comprised substantially or completely of Co R intermetallic phase where R is samarium, neodymium, or preferably, a mixture of samarium and neodymium. Generally, the present base alloy is comprised of about to 68 percent by weight cobalt and about 32. to 35 percent by weight rare earth metal or metals. Although the base alloy may vary in composition, it should have a composition which together with the sintering additive, produces the claimed composition of the present sintered product.

The present sintering additive is a cobalt-rare earth metal alloy which is richer in'rare earth metal content than the base alloy. Preferably, it is also one that exists at least partly in a liquid form at sintering temperature, but it can be a solid. The present sintering additives are alloys of cobalt-samarium, cobalt-neodymium, or cobalt-samarium-neodynium.

The sintering additive alloy may vary in composition and can be determined from the phase diagram for the particular cobalt-rare earth system or it can be determined empirically. When liquid phase sintering is desired, FIG. I shows that for the cobalt-samarium system, for example, there are phases which are partly or completely liquid at the temperature ranging from about 950 to 1,200C. Any alloy within the range shown in FIG. 1 which forms at least a partly liquid phase at the particular sintering temperature would be a satisfactory additive. For example, as illustrated in FIG. 1, the Co-Sm additive alloy can vary upward in samarium content from about 46 percent by weight-of the additive.

When a sintering additive which is solid at sintering temperature is desired, it also may vary in composition and can be determined from the phase diagram for the particular'cobalt-rare earth system or which can be de- 'termined empirically. For example, FIG. 1 shows that for the cobalt-Samarium system, there is a solid phase containing samarium in an amount greater than about 36 percent by weight at a temperature ranging from 950 to l,200C. Specifically, from a temperature of 950 to l,075C,' the solid additive alloy for the cobaltsamarium system ranges in samarium content from about 36 to about 55 percent by weight of the additive, and at temperatures ranging from 950 to l200C, the solid additive alloy mayrange in samarium content from about 36 percent to about 45 percent by weight of the additive. Any additive ailoy within these ranges would be a satisfactory sintering additive alloy.

If desired the sintering additive can be empirically selected by a number of methods, such as by means of a composition scan at the sintering temperature, i.e., heating samples of various additive alloy compositions to the desired sintering temperature to determine which is solid and which is at least partly liquid at sintering temperatures.

Although suitable sintering additive alloys fall within a general composition range, the preferred ones are comparatively low in rare earth metal content so that undesirable characteristics of the pure rare earth metal in the additive alloy are minimized. Specif cally, for example, pure samarium is both pyrophoric and very ductile and consequently difficult to crush and to blend with the base alloy since it has a tendency to separate out and fall to the bottom of the container. However, a sintering additive Co-Sm alloy of Samarium content preferably less than 70 percent by weight is substantially non-reactive at room temperature in air, it can be crushed by conventional techniques, and being slightly magnetic, it clings to the base alloy resulting in a substantially thorough stable mixture. The higher the cobalt content of the additive alloy, the stronger are its magnetic properties and the more stable is the particulate mixture it forms with the base alloy.

In preparing the present sintered product, the base and sintering additive cobalt-rare earth alloys can be formed by a number of methods. For example, each can be prepared by arc orinduction melting the cobalt and rare earth metal together in the proper amounts under a substantially inert atmosphere such as argon and allowing the melt to solidify. Preferably, the melt is cast into an ingot.

The solid base and additive alloys can be converted to particulate form in a conventional manner. Such conversion can be carried out in air at room temperature since the alloys are substantially non-reactive. For example, each alloy can be crushed by mortar and pestle in air and the pulverized to a finer form by fluid energy milling in a substantially inert atmosphere.

The particle size of the base and additive cobalt-rare earth alloys used in forming the present mixture may vary. Each can be in as finely divided a form as desired. For most applications, average particle size will range from about 1 micron or less I to about microns. Larger sized particles can be used, but as the particle size is increased, the maximum coercive force obtainable is lower because the coercive force generally varies inversely with particle size. In addition, the smaller the particle size, the lower is the sintering temperature which may be used.

In forming the present mixture, the base and sintering additive alloys are each used in an amount so that the resulting mixture has a cobalt and rare earth metal content substantially corresponding to that of the final desired sintered product. In addition, however, in forming the mixture, the sintering additive should be used in an amount sufficient to promote sintering. This amount depends largely on the specific composition of the additive and can be determined empirically, but generally, the sintering additive alloy should be used in an amount of at least 0.5 percent by weight of the baseadditive alloy mixture. Generally, for liquid phase sintering, the larger the rare earth metal component of the sintering additive alloy, the more liquid it is at sintering temperature. Specifically, for liquid phase sintering, a sintering additive composed of 40 percent Co and 60 percent Sm may generally be used, and preferably in an amount ranging from about 4 to 25 percent by weight of the base-additive alloy mixture.

In carrying out the process of this invention, the base alloy is admixed with the additive alloy in any suitable manner to produce a substantially thorough particulate mixture. The particulate mixture can then be compressed into a green body of the desired size and density by any of a number of techniques such as hydrostatic pressing or methods employing steel dies. The mixture is compressed in the presence of an aligning magnetizing field to magnetically align the particles along their easy axis, or if desired, the mixture may be compressed after magnetically aligning the particles. The greater the magnetic alignment of the particles, the better are the resulting magnetic properties. Preferably compression is carried out to produce a green body with as high a density as possible, since the higher its density, the greater the sintering rate. Green bodies having a density of about 40 percent or higher of theoretical are preferred.

The green body is sintered to produce a sintered body of desired density. Preferably, the green body is sintered to produce a sintered body wherein the pores are substantially non-interconnecting. Such non-inter.- connectivity stabilizes the permanent magnet properties of the product because the interior of the sintered product or magnet is protected against exposure to the ambient atmosphere.

The sintering temperature used in the present process may vary. The minimum sintering temperature must be sufficiently high for sintering to occur in a particular cobalt-rare earth system, i.e., it must be high enough to coalesce the component particles. In the present process, a sintering temperature of about l,O50C to l, l 50C is suitable with a sintering temperature of l,l00C to l,l25C being particularly satisfactory. I

Preferably, sintering is carried out so that the pores in the sintered product are substantially noninterconnecting. A sintered body having a density or packing of at least about 87 percent of theoretical is generally one wherein the pores are substantially noninterconnecting. Such non-interconnectivity is determinable by standard metallographic techniques, as for example, by means of transmission electron micrographs of a cross-section of the sintered product. The maximum sintering temperature is preferably one at which significant growth of the component particles or grains does not occur, since too large an increase in grain size deteriorates magnetic properties such as ooercive force. The green body is sintered in a substantially inert atmosphere such as argon, and upon completion of the sintering, it is preferably cooled to room temperatures in a substantially inert atmosphere.

ranging down to about 80 percent may be satisfactory.

The preferred density of the sintered product is one which is the highest obtainable without producing a growth in grain size which would deteriorate magnetic properties significantly, since the higher the density the better are the magnetic properties. For sintered products of the present invention, a density of at least about 87 percent of theoretical, i.e., of full density, and as high as about 96 percent of theoretical is preferred to produce permanent magnets with suitable magnetic properties which are substantially stable.

Sintering of the green body produces a sintered prod uct which weighs about the same as the green body indicating no loss, or no significant loss, of cobalt and rare earth components. Standard chemical analysis of a sintered product shows that the rare earth and cobalt content is substantially unaffected by the sintering process.

The magnetic properties of the present sintered products can be improved by subjecting them to a heataging process to produce novel permanent magnets. This heat-aging process is substantially disclosed in US. Pat. No. 3,684,593, which is a continuation-inpart of copending US. Pat. application Ser. No. 33,315, now abandoned entitled Heat-Aged Sintered Cobalt-Rare Earth Intermetallic Product and Process," filed Apr. 30, 1970 in the names of Mark G. Benz and Donald L. Martin and assigned to the assignee hereof, and which by reference is made part of a disclosure of the present application.

As applied to the preparation of the new permanent magnets of the present invention, the heat-aging pro cess comprises heating the present sintered products at a temperature within 400C below its sintering temperature for a period of time ranging up to 24 hours in a substantially inert atmosphere such as, for example, argon. The particular aging temperature and aging period is determinable empirically depending on the resulting improvement in magnetic properties produced.

Magnetization of the present sintered products of cobalt, samarium and neodymium produces permanent magnets with magnetic properties which make them useful for a wide variety of applications. One particular advantage of the present invention is that neodymium is a much more abundant element than samarium, thereby making the present permanent magnets available for a wider variety of applications than has been possible heretofore. Also, neodymium provides potentially the highest saturation induction.

The permanent magnets of the present invention are substantially stable in air and have a wide variety of uses. For example, they are useful in telephones. electric clocks, radios, television, and phonographs. They are also useful in portable appliances,.such as electric toothbrushes and electric knives, and to operate automobile accessories. ln industrial equipment, the present permanent magnets can be used in such diverse applications as meters and instruments, magnetic separators, computers and microwave devices.

If desired, the sintered bulk product of the present invention can be crushed to a desired particle size preferably a powder, which is particularly suitable for alignment and matrix bonding to give a stable permanent magnet. The matrix material may vary widely and may be plastic, rubber or metal such as, for example, lead, tin, zinc, copper or aluminum. The powdercontaining matrix can be cast, pressed or extruded to fonn the desired permanent magnet.

All parts and percentages used herein are by weight unless otherwise noted.

The invention is further illustrated by the following the sintered product was cooled in the same purified argon atmosphere.

Percent packing was determined from the measured density of the sample divided by the full density of the alloy under consideration. The full alloy densities used are as follows:

B is the saturation induction.

B, is the residual or remanent induction, i.e., the flux when the applied magnetic field is reduced to zero.

Normal coercive force H, is the field strength at which the induction B becomes zero.

The maximum energy product (Bl-U represents the maximum product of the magnetic field H and the induction B determined on the demagnetization curve.

411'] is magnetization.

EXAMPLE In the runs of the following table, each alloy melt was made under purified argon by induction melting and cast into an ingot. The ingot was then crushed in air by means of mortar and pestle or in a jaw crusher in nitrogen and then ground in nitrogen by fluid energy milling into a powder of 6 to 8 microns average particle size. The sintering alloy was admixed with the base alloy by tumbling to form a substantially thorough mixture which was stable since the additive was substantially non-reactive in air and was slightly magnetic. ln Runs 1-4 of the following table, the composition of the base alloy was 66.7 percent cobalt-16.6 percent samarium- 16.6 percent neodymium, the composition of the addi' tive alloy was 40 percent cobalt-60 percent samarium,

and the base and additive alloys were each used in an amount to give the green body eompositionshown in the following table. In Run No. 5 of the following table,

the composition of the base alloy was 68 percent cobalt-32 percent samarium and the composition of the additive alloy was 40 percent cobalt-60 percent samarium and the magnetic properties were determined after the entire treatment was completed.

The green body of Run Nos. 1 through was formed by packing the mixture into a rubber tube having a working space of inch in diameter and l-% inch long. The tube was placed in an axial magnetic field of 60,000 oersteds to align the particles along the easy axis. After aligning, the powder was compressed and the sample was subsequently hydrostatically pressed under 200,000 psi. The pressed samples, i.e., green bodies, had a packing density of about 80 percent. The green bodies were cylindrical in form and had a diameter ranging from about Mr up to about inch and a length ranging from about 1 to l-Vz inches. Generally, the green bodies were machined to make a right cylinder of proper dimensions for testing purposes. The green bodies were treated as indicated in the following table. The magnetic properties of Runs l-5 were determined after magnetization at room temperature in a field of 60,000 oersteds.

Tn eopending U.S."Patfapplieation'SETNo. 66,173 now U.S. Pat. No. 3,682,715 entitled Sintered Cobalt- Rare Earth lntermetallic Product Including Samarium And Lanthanum And Permanent Magnets Produced Therefrom filed on Aug. 24, 1970 in the name of Donald L. Martin and assigned to the assignee hereof, there is disclosed novel sintered products comprised of intermetallic compounds of cobalt and rare earth metals composed of samarium and lanthanum and permanent magnets produced therefrom.

In eopending U.S. Pat. application Ser. No 66,172 now U.S. Pat. No. 2,684,591 entitled Sintered Cobalt- Rare Earth lntermetallic Product Including Samarium and Cerium And Permanent Magnets Produced Therefrom" filed on Aug. 24, 1970 in the name of Donald L. Martin and assigned to the assignee hereof, there is disclosed novel sintered products coniised of intermetallic compounds of cobalt and rare earth metals composed of samarium and cerium and permanent magnets produced therefrom.

In eopending U.S. Pat. application Ser. No. 66,216 now U.S. Pat. No. 3,682,716 entitled Sintered Intermetallic Product of Cobalt, Samarium And Cerium Green Body Sintered Product (BI-113 ",r Run Co Sm Nd Ali n- Den Packin B, B, HC :auss No. me n Treatment sity L Gauss Gauss Oers. Oers.)

1 62 19 19 .92 a A Pressed 6.88 81.0 9.1 6.7 1.6 5 .956 b 1 in" at 1100 C 7.62 89.7 9.1 7.8 3.4 10 .944 c 1 hr at 110 C. tlren Furnace Cooled 7.72 90.8 9.3 7.96 5.0 14

to 900 .tg n 1' hrat 900C 2 62.5 18.75 18.75 .88 a As Presse 6.85 80.6 8.06 6.1 1.5 4

.917 b 1 hr at 110 7.63 89.8 9.12 7.5 3.4 10.5 .926 c 1 hr at 110C.t en Furnac Cooled 7.93 93.3 9.10 7.86 5.2 14

to 900 tgen 1 2 hr at 900 C 3 63 18.5 18.5 .915 a A Presse 6.80 80.0 9.37 6.8 1.6 5 .948 b 1 rat 110 C 7.62 89. 9.57 8.13 2.9 11 .950 c lhrat 110C.t en Furnace (.ooled 7.92 93.2 9.71 8.59 5.0 16

to 900 then 1 2 hr at 900 C 4 63.5 18.25 18.25 .8 4 a As'Pressed 6.82 80.2 9.20 6.3 1.6 4.5

.929 b 1 hr at 110 C 7.64 89.8 9.45 7.9 3.8 12 .936 c 1 hr at 110 C.t en Furnace C ooled 7.93 93.3 9.6 8.4 5.9 17 5 62 s 37 5 9s 1 ti t 1 f h 18668? 93 9 1 s 1 6 2 16 0 r a r urnace Cooled to 900 then 16 hrs at 900 C "R1111 Nos. l thFo u gh 4 illustrate the properties of the permanent magnets of the present invention. Specifically, the present magnets have permanent magnet properties which make them useful for a wide variety of applications as, for example, meters.

FIG. 2 shows what positive yalues of magnetization 41rJ can be maintained in the presence of the demagnetizing field H for Run No. 4.

Copending U.S. Patent Application Serial No. 33,2- 24 now U.S. Pat. No 3,695,945 entitled Sintered Cobalt-Rare Earth Intermetallic Product," filed on Apr. 30, 1970, in the name of Mark G. Benz and assigned to the assignee hereof is, by reference, made part of the disclosure of the present application. In U.S. Pat. application Ser. No. 33,224, there is disclosed a process which, briefly stated, comprises forming a particulate alloy of cobalt and rare earth metal content substantially the same as that desired in the sintered product. compressing the particulate alloy into compacts and sintering them to the desired density. The sintered product is comprised of a major amount of Co R intermetallic phase and up to about percent by weight of the product of a second CoR phase which is richer in rare earth metal content than the Co R phase, where R is a rare earth metal. These sintered products are then magnetized to form novel permanent magnets.

Mischmetal And Permanent Magnets Produced Therefrom filed on Aug. 24, 1970 in the names of Donald L. Martin and Mark G. Benz and assigned to the assignee hereof, there is disclosed novel sintered products comprised of intermetallic compounds of cobalt and rare earth metals composed of Samarium and cerium mischmetal and permanent magnets produced therefrom.

1n eopending U.S. Pat. application Ser. No. 66,171 now U.S. Pat. No. 3,682,714, entitled Sintered Cobalt-Rare Earth lntermetallic Product And Permanent Masasa lr dss d lhsrr q nAus-li 1970 in the name of Donald L. Martin and assigned to the as signee hereof, there is disclosed novel sintered products comprised of intermetallic compounds of samarium and praseodymium and permanent magnets produced therefrom.

All of the above cited patent applications are, by reference, made part of the disclosure of the present application.

What I claim as new and desire to secure by Letters Patent of the United States is:

i A permanentrnagnet consisting essential lyof a sintered product of compacted particles consisting essen- 9 hase alloy \i/here li a'xari are seraea'rrzaatn 'fiip consisting of samarium, neodymium and alloys thereof,

said sintered product ha ing pore siiriii c h are substaii tially non-interconnecting, a packing of at least 87 percent and a composition consisting essentially of cobalt in an amount of 62 to 66 percent by weight of said sintered product'and the rare earth metals of Samarium and neodymium in an amount of 34 to 38 percent by weight of said sintered product with'the neodymium 

